Unless otherwise indicated herein, the materials described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
Increasing carbon dioxide (CO2) gas in the atmosphere possibly affects environmental changes. For example, the increase of CO2 gas is known to cause carbonation or neutralization of concrete when the hydrogen ion concentration becomes pH 9 or less.
Due to a porous structure of concrete with numerous pores and voids, moisture, chloride ions, carbon dioxide, oxygen, etc., may permeate throughout the concrete by capillary action through the pores and voids. Normal concrete, which is strongly alkaline, is neutralized by the reduction of hydroxide (OH−) ions due to permeation of CO2 gas and chloride compounds from salinity corrosion. This neutralization of concrete does not directly cause a decrease in the strength of the concrete itself. However, reinforcing steel for supporting various structures contained in the concrete can be severely damaged. For example, when CO2 gas and chloride (Cl−) ions in the atmosphere reach the reinforcing steel inside the concrete from the surface, a passivation layer, typically having the thickness of approximately 3 mm, on the surface of the reinforcing steel can be corroded, resulting in rust on the surface of the reinforcing steel. These corrosions and rust can cause the expansion of the volume of the passivation layer on the reinforcing steel by approximately two to three times. The expansion of the volume of the passivation layer burdens the surrounding concrete with extra stress, thus the strength of the whole concrete structure deteriorates due to the extra stress.
To avoid the deterioration of the concrete structure, a prestressing cable reinforcement method has been recently employed. In this method, prestressed concrete with a predetermined level of pressure applied in advance, and various reinforcing cables attached to the exterior of the prestressed concrete are employed instead of reinforcement materials, such as reinforcing steel, to be contained in the concrete structure. In the prestressing cable reinforcement method, it is desirable to ensure the long-term integrity of the installed reinforcing cables by establishing a maintenance method for monitoring the cable integrity.
The reinforcing cables can be made of various types of steel, surrounded by protective coatings of various plastic materials, such as polyethylene, in order to prevent corrosion of the passivation layer due to permeation of CO2 and/or Cl− and possible degradation due to exposure to the outdoor atmosphere after installation. However, these plastic materials may have cracks in the protective coating as a result of many different natural causes that accelerate deterioration, such as heat, moisture, ultraviolet rays from the sun and oxidation due to nitrogen oxide (NOR) gas or sulfur oxide (SOx) gas in the atmosphere, as well as human-induced causes, such as mechanical damage. Thus, there is a need for monitoring the integrity of the installed reinforcing cables without destroying or otherwise compromising the protective plastic coating.